As herein described, reference to and the use of the terms surface acoustic wave (SAW) and SAW device are intended for any device using the propagation of elastic waves on the surface of a material or at the interface of several materials. It is to be understood that disclosure herein described may be applied to different kinds of elastic waves as long as they can be generated or detected using interdigital transducers (IDTs). For example, so called Leaky SAWs, Pseudo SAWs, Boundary Waves, Surface Transverse Waves, Interface Waves, or Love Waves are considered herein to be SAWs.
As is well known in the art, surface acoustic devices use so called interdigitated transducers (IDTs) to transform electric energy to acoustic energy, or reciprocally acoustic energy to electric energy. By way of example, the IDT illustrated with reference to FIG. 1 uses a piezoelectric substrate and two opposing busbars at two different electrical potentials and two sets of electrodes connected to the two busbars. Due to the piezoelectric effect, the electrical field between two successive electrodes at a different potential provides an acoustical source.
Reciprocally, if the transducer receives an incoming wave, charges are generated in the electrodes as a result of piezoelectric effects. A resonator is obtained by placing a transducer between two reflective gratings as illustrated with reference to FIG. 2. Filters or duplexers can be designed by connecting several resonators or by having one or several transmitting IDTs generating acoustic energy, this acoustic energy being received by one or several IDTs.
One typical problem when designing surface acoustic wave (SAW) devices, mostly on quartz, involves the elastic wave velocity in the transducer region being slower than the velocity in the busbars region. The transducers perform as a waveguide preventing the leaking of acoustic energy from the transducer and help to reduce losses. However, when this waveguide supports more than one guided mode, the device transfer function presents undesired ripples or spurii. This is generally addressed in several ways.
One simple way is to choose an acoustic aperture small enough to have only one guided mode. This may result in an excessive load or undesirable source impedances for the device. Another way includes use of an apodization of the transducer in order to try to match the transverse profile of the modes. This also results in undesirably large impedances. The use of 2-D periodic obstacles is yet another way to reduce the transverse modes, but it implies a more complicated manufacturing process. The piston mode approach relies on a change of velocity profile in the transducer in order to have one propagating mode having an essentially flat shape in the transducer aperture. This approach was described for example in U.S. Pat. No. 7,576,471, the disclosure of which is herein incorporated by reference in its entirety, for a case where the velocity is lower in the transducer than in the bus bars. In addition, due to the smaller velocities difference, the gap region has a minor impact.
For wideband devices, a high piezoelectric coupling material such as Lithium Niobate or Lithium Tantalate has to be used. In this case, the transducer configuration can be different than the usual configuration on quartz. If the velocity of the acoustic wave in the busbars is slower than the velocity of the wave in the transducer, it no longer performs as a desirable waveguide, thus transverse modes are no longer possible. This also allows the acoustic energy to leak outside the transducer and can result in losses.
In practice, the situation is more complicated. For high coupling substrates, the electrical conditions at the surface have a large impact on the velocity and the velocity in the electrode end gaps is usually much larger than the velocity in the transducer aperture and larger than the velocity in the busbars. The length of the gaps is usually of the same order of magnitude as the electrode width, typically a fraction of the acoustic wavelength. In this case, both transverse modes due to the reflections on the edge gaps and energy leaking outside the transducer result. The velocity difference between the transducer region and the gap region is large enough to have a full reflection on the edges while the edge gap is small enough so that some energy is leaking outside by a tunnel effect.
To suppress the unwanted transverse modes, one typical method includes use of apodization, as illustrated with reference to FIG. 3. In this case, the position of the edge gap extends into the transducer aperture region. Since the position of the gap has a large impact on the modes, the mode shapes are varying along the transducer length. As a result, undesired transverse modes occur at different frequencies and their desired effect is reduced.
Similarly, Ken Hashimoto in [T. Omori, †K. Matsuda, Y. Sugama, †Y. Tanaka, K. Hashimoto and M. Yamaguchi, “Suppression of Spurious Responses for Ultra-Wideband and Low-Loss SAW Ladder Filter on a Cu-grating/15°YX-LiNbO3 Structure”, 2006 IEEE Ultrasonics symp., pp 1874-1877] presented a transducer where the gap position is constant while the aperture is changing in the transducer, as illustrated with reference to FIG. 4. This may be referred to as dummy electrodes apodization. This transducer is working by changing the transverse modes frequencies along the transducer.
By way of further example, a patent application of Murata [US2007/0296528A1] describes a SAW transducer that has wider electrodes in front of the edge gap to try to reduce the velocity difference between the edge gap region and the transducer aperture region, as illustrated with reference to FIG. 5. Another Murata patent application [US2008/0309192 A1] discloses a modified version of the apodization using a function with more than one extreme, as illustrated with reference to FIG. 6. Performance characteristics including phase and impedance for such are illustrated with reference to the plots of FIG. 6a. 
SAW transducers often use so called “dummy electrodes” as further illustrated with reference again to FIG. 3. These dummy electrodes are used to suppress a velocity difference between the active region of the transducer and the inactive region of the transducer, especially when apodization is used.
Typically, the electrode end gap separating the dummy electrode from the active electrode is chosen in the order of magnitude of the electrode width (a fraction of wavelength) in order to reduce its effect as much as possible. When a high coupling material is chosen, the velocity in the gap is much higher than the velocity in the transducer. In this case, even if the gap length is small, it is found that the gap position has a very large impact on the transverse modes.
All these teachings try to reduce undesirable effects of the edge gap of the transducer. Even if good quality factors were demonstrated, the apodization results in an undesirable reduction of the equivalent coupling coefficient. In addition, the wave velocities are such that wave guiding is not possible for the transducer, and otherwise useful energy leaks outside the transducer.